The efficient oxygenation of the hydrocarbons has enormous commercial value for the production of commodity chemicals and for many important products made therefrom. For example, nylon is one such product which has enormous commercial applications and which is made from starting materials, i.e. cyclohexane, which must first be oxidized into alcohol and ketone intermediate compounds during the manufacture of nylon. The oxygenation of cyclohexane, however, is extremely expensive and inefficient, having a conversion of only 3-4%. Sufficient oxygenation of various saturated hydrocarbons has long been a difficult challenge which has provoked the search for efficient and cost effective oxidation catalysts and processes for use in the chemical industry.
Toward this end, transition metal catalysts have been studied for some time for their use as oxidation catalysts for saturated hydrocarbons. More recently, the use of metallic porphyrin complexes have been reported as effective oxidation catalysts using certain substrates, oxidants and process parameters. For example, U.S. Pat. No. 4,822,899 to Groves et al., describes the use of metallic porphyrin complexes as catalysts in epoxidation reactions. The process disclosed in this reference involves adding to a solution of metallic porphyrin catalyst an olefin or mixture of olefins to form a reaction solution. The reaction solution is purged with an oxidizing agent, e.g. commercially pure oxygen or air, and the reaction is conducted at ambient conditions for 24 hours. The results after 24 hours shows a maximum turnover rate of 45.4, as shown in Example 8, in the Table, Column 5. The turnover rate is the number of molecules of oxidized product turned out per each molecule of catalyst for a given time period. Thus, while U.S. Pat. No. 4,822,899 demonstrated the ability to use metallic porphyrin complexes as oxidation catalysts using air as the oxidant and under ambient conditions, the efficiency of such reactions even under the best of reaction conditions, e.g. under optimum temperature and pressure parameters, is comparatively low. That is, the turnover rate and yields represented an improvement over previous efforts but did not approach the level useful for commercial scale-up.
Subsequent efforts using metallic porphyrins as oxidation catalysts for olefins, alcohols, sulfides and alkanes using heteroaromatic N-oxides, demonstrated that the addition of strong mineral acids such as HCl and HBr enhances oxidation selectively and efficiently. See Ohtake et al. Heterocyles, Vol. 40, No. 2, pp. 867-887 (1995); Ohtake et al., J. Am. Chem. Soc., 114, 10660-10662 (1992). These references suggest that halogens coordinate as an axial ligand and accelerate the deoxygenation of N-oxide by ruthenium porphyrin. The use of such acids, however, may decompose some of the end products and result in halogen by-products which require appropriate separation and handling. Moreover, certain aromatics, such as benzene, were reported by Ohtake et al. as not being useful oxidative substrates. Additionally, the highest turnover rate described in Ohtake et al. over a period of 24 hours with the addition of these mineral acids was shown as 12,500. See J. Am. Chem. Soc., 114, 10661 (1992).
The use of a nitro group on a coordinated ligand has been reported in the literature. See Sheldon R. A.; Kochi J. K., Metal-Catalyzed Oxidation of Organic Compounds, p. 112-113, Academic Press (1981). The nitro group, however, is used to transfer oxygen to a substrate, i.e. to oxidize the substrate. This reference shows that the oxidation state of the metal does not change. This is in sharp contrast to the present invention where the nitrosyl group present in the porphyrin or salen ligand structure remains intact and the oxidation state of the metal center changes to the highly active M.sup.III species.
U.S. Pat. No. 4,900,871 to Ellis, Jr. et al., discloses iron coordination complexes containing a halogenated ligand. The iron porphyrin complex is disclosed as having Fe.sup.II and Fe.sup.III states and the improvement in oxidation of alkanes by contact with air or oxygen is reported as being due to the introduction of halogen into the coordination complex. Oxidation of propane using such complexes show a maximum turnover of 583.1 per 1.5 hours at 150.degree. C. and a maximum turnover of 849 per 6 hours at 80.degree. C. using isobutane as the substrate. This patent states in column 7, line 29, that halogenating the ligand of the iron coordination complex improves activity but has little or no effect on activity where the metal is other typical transition metals such as manganese or chromium. Furthermore, these porphyrins have been reported as being free radical initiators rather than catalysts per se. See Grinstoff, M. W.; Hill, M. G.; Labinger, J. A.; Gray H. B., Science, 1994, 264, 1311-1313.
U.S. Pat. No. 5,280,115 to Ellis, Jr. et al. discloses metal complexes of porphyrin catalysts for the oxidation of alkanes where the metal may be ruthenium. The porphyrin ring has nitro groups attached thereto in the meso and/or .beta.-pyrrolic positions.
U.S. Pat. No. 4,895,680 to Ellis, Jr. et al. discloses nitride-activated metal coordination complexes such as M-X where M is a transition metal and X is a nitride on the oxidation of alkanes. The turnovers/hour (T.O./hr.) using nitride-activated Fe, Cr and Mn porphyrins are relatively low. For example, oxidation of cyclohexane to cyclohexanol and cyclohexanone using the nitride-activated catalysts showed T.O./hr. values of 33 and 23 and 27 for Cr(TPP)N, Mn(TPP)N and [Fe (TPP)].sub.2 N respectively.
Also present in the literature is a report on the theoretical existence of a ruthenium IV (Ru.sup.IV) porphyrin radical species derived from a Ru.sup.III precursor. See Mlodnicka, T., James, B. R., Metalloporphyrin Catalyzed Oxidations, 121-148, Klewer Academic Publishers, (1994). This reference used a phosphine ligand and iodosylbenzene as an oxidant to achieve low oxidation product yields and low turnovers on hydrocarbon substrates such as cyclohexane, styrene, norbornene and stilbene (see Table 1, p. 131). The experiments reported therein showed turnovers of 1.3-130/6 hrs. It should be mentioned that such low catalytic results occurred notwithstanding the use of norbornene, a relatively reactive substrate, a halogenated porphyrin and in the presence of an exotic non-commercial oxidant iodosylbenzene.
The present invention represents a significant improvement over the prior art oxidation processes which employ metallic porphyrin complexes as oxidative catalysts. The efficiency, selectivity and speed have been significantly improved over the prior art without the use of mineral acids, and such difficult substrates as cyclohexanes and benzene have been oxygenated efficiently using the present invention. Thus, the present invention addresses the long sought need of finding a means to rapidly and efficiently oxidize commercially important substrates such as olefins, alkanes, aromatics, and alcohols (aliphatic or aromatic) by carefully designing the porphyrin or salen complex and the associated ligands. The coordination complexes of the present invention are designed by selecting substituents, both on the external structure and the internal ligand set, such that the maximum activity of the catalytic metal can be obtained. The choice of substituents is dictated by the need to control the electronic structure of the catalyst to prevent or minimize the formation of a stabilized ligand set adjacent the metal, as well as to minimize or prevent the formation of stabilized electron configurations on the metal per se a phenomenon known as disproportionation.